Usage of inline filters and high frequency attenuation cables has sharply increased over the past few years, especially in military applications to prevent accidents where, for example, electromagnetic interference causes bomb bay doors to open and landing gear to engage without pilot command.
Various attempts have been made to solve these and other problems associated with electromagnetic interference by using high frequency attenuation cables. For instance, it has long been known in the art that wrapping a core conductor with a high frequency absorption medium such as lossy material causes the low frequency energy to pass through the cable unobstructed, while the high frequency energy is absorbed in the lossy layer. This is specifically set forth in F. Mayer's U.S. Pat. Nos. 3,309,633 and 3,191,132.
Many embodiments of a lossy high frequency attenuation cable have been previously disclosed by others. For instance in Murphy, U.S. Pat. No. 3,215,768 a conductor is wrapped by combinations of low and high permeability shields and then wrapped by an insulator. The same basic structure is set forth in Hirose, U.S. Pat. No. 3,683,309 wherein a textile filament supports a lossy layer, the lossy layer is wrapped by a conductive non-metallic filament and surrounded in turn by a second lossy layer, rubber insulation surrounds the second lossy layer, the elements recited above are then surrounded by a protective rubber coating. Also, Clark, U.S. Pat. No. 3,219,951 shows a similar structure wherein the external jacket is of a non-conducting or insulating material.
The art of high frequency attenuation cables has further been advanced through the development of particular lossy mediums. For instance, F. Mayer in U.S. Pat. No. 3,191,132 describes particular ferrite compounds for his lossy layers, in Fondiller, U.S. Pat. No. 1,672,979 the lossy layer includes finely divided particles of pure iron, see also F. Mayer, U.S. Pat. No. 3,309,633 where particular types of lossy layers are disclosed. Those portions of the above cited disclosures referring to particular lossy mediums are incorporated herein by reference.
In previously known high frequency attenuation cables the high frequency attenuation was often lower than desired. Applicant herein suggests that previously known high frequency attenuation cables may have suffered from having alternate paths by which the high frequency energy could travel from one end of the cable to the other without significant attenuation. These paths are referred to as sneak paths. Applicant provides additional conductive material on the outside of the cable which eliminates these sneak paths.
Others have suggested wrapping the lossy layer with an outer layer of conductive material. For instance, in F. Mayer, U.S. Pat. No. 4,104,600 which discloses a lossy core supported by an inside textile thread, the lossy core being wrapped by a conductor and a second lossy layer which includes dielectric material, the second lossy layer is designed to serve as both a dielectric and a high frequency energy absorber, and the above recited elements surrounded by a conductive outer sheet. Additionally, British Insulated Cables, Limited in British Pat. No. 565,228 (hereinafter BIC) discloses a textile core surrounded by a lossy layer which is in turn surrounded by a conductor, the lossy layer is in turn surrounded by a conductive rubber or ozone-resistant material, followed by a layer or layers of insulating material, which are in turn followed by a protective layer and a tubular metallic braid which may form the external covering of the cable or may be itself enclosed in a protective layer. Note BIC also suggests using a protective covering (assumed to be non-conductive around the above recited elements (p. 2, lines 75-77). Another example of additional conductive material in a high frequency attenuation cable is found in Schlicke et al, U.S. Pat. No. 3,541,473 (hereinafter Schlicke et al) which discloses a twin conductor power cable having one of the conductors as the outer most layer with lossy material and dielectric between the two conductors.
Applicant's structure differs from previous structures inasmuch as applicant provides a shielding layer and an outer conductive jacket around a core conductor surrounded by a high frequency absorbing medium and a dielectric. The shielding layer prevents electromagnetic interference and in combination with the outer conductive jacket, as will be appreciated more fully hereinafter, retains the high frequency energy in the lossy layer by blocking any alternate path which could develop if the outer conductive jacket were an insulator. In comparison to F. Mayer, U.S. Pat. No. 4,104,600, applicant provides a structure which both shields the cable against electromagnetic interferences (hereinafter EMI) at the same time retaining the high frequency energy in the lossy material. In comparison to BIC or Schlicke, et al, applicant provides a high frequency attenuation cable which does not allow the shielding found in the above cited references to act as a high frequency transmission line.
Typical of the commercial art of high frequency attenuation cables against the background of the above recited references in Capcon Inc.'s (147 West 25th St., New York, N.Y. 10001) lossy line filter cable which includes a conductor surrounded by a lossy medium which is in turn surrounded by dielectric and in turn surrounded by shielding braids and finally surrounded by an outer insulating jacket. Applicant has developed a commercial high frequency attenuation cable different from any high frequency attenuation cable heretofore which increases the amount of filtering heretofore known as well as ensuring the elements comprising the cable function properly.